Generally, MIMO (multi-input multi-output) technique means a method of improving transceived data efficiency by adopting multiple transmitting antennas and/or multiple receiving antennas instead of a single transmitting antenna and a single receiving antenna. Namely, the MIMO technique is the technique for a transmitting or receiving side of a wireless communication system to attempt capacity increase or performance enhancement using the multiple antennas.
The MIMO technique is the technique for receiving one whole message in a manner that pieces of data segments received via plural antennas are put together to complete the whole message. The MIMO antenna improves a data transmission rate within a specific range or extends a system range for a specific data transmission rate, thereby being widely applicable to a mobile communication terminal, a relay and the like. Many attentions are paid to the MIMO technique as a next generation technology capable of overcoming the transmission size limit of the mobile communication which almost reaches a dead end due to data communication expansion and the like.
FIG. 1 is a diagram for a configuration of an MIMO communication system according to a related art. Referring to FIG. 1, if the number of transmitting antennas is incremented into NT and the number of receiving antennas is simultaneously incremented into NR, theoretical channel transmission capacity is increased in proportion to the number of antennas unlike the case that a transmitter or receiver uses a plurality of antennas. Hence, a transmission rate is enhanced and frequency efficiency can be remarkably raised. The transmission rate according to the increase of the channel transmission capacity can be theoretically raised in a manner that a maximum transmission rate R0 of the case of using a single antenna by a rate increasing rate Ri of Formula 1.Rt=min(NT,NR)  [Formula 1]
For instance, an MIMO communication system, which uses 4 transmitting antennas and 4 receiving antennas, is able to obtain a transmission rate 4 times higher than that of a single antenna system. After this theoretical capacity increase of the MIMO system has been proved in the middle of 90's, many efforts are ongoing to be made to various techniques to drive it into substantial data rate improvement. Some of theses techniques are already adopted as standards for the 3G mobile communications and various wireless communications such as a next generation wireless LAN and the like.
The trends for the MIMO relevant studies are explained as follows. First of all, many efforts are ongoing to be made in various aspects to develop and research information theory study relevant to MIMO communication capacity calculations and the like in various channel configurations and multiple access environments, radio channel measurement and model derivation study for MIMO systems, spatiotemporal signal processing technique study for transmission reliability enhancement and transmission rate improvement and the like.
In order to explain a communicating method in an MIMO system in detail, mathematical modeling can be represented as follows.
First of all, a transmission signal is explained. If there are NT transmitting antennas, NT maximum transmittable informations exist. Hence, it can be represented by the vector shown in Formula 2.s=└s1,s2, . . . ,sNT┘T  [Formula 2]
Meanwhile, transmission power can be set different for each transmission information. If the respective transmission powers are set to P1, P2, . . . , PN, the transmission power adjusted transmission information can be represented by Formula 3.ŝ=[ŝ1,ŝ2, . . . ,ŝNT]T=[P1s1,P2s2, . . . ,PNTsNT]T  [Formula 3]
And, the transmission power adjusted transmission information vector can be represented by Formula 4 on a transmission power diagonal matrix P.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Formula          ⁢                                          ⁢          4                ]            
If the transmission power adjusted transmission information vector is multiplied by a weight matrix W, NT transmission signals are generated. In this case, the weight matrix is the matrix that plays a role in properly distributing each transmission information to each antenna according to a channel status and the like and is also called a precoding matrix.
If the NT transmission signals are set to x1, x2, . . . , xNT, a transmission signal vector x can be represented by Formula 5. In Formula 5, wij indicates a weight between an ith transmitting antenna and a jth information.
                    x        =                                  ⁢                              [                                                  ⁢                                                                                x                    1                                                                                                                    x                    2                                                                                                ⋮                                                                                                  x                    i                                                                                                ⋮                                                                                                  x                                          N                      T                                                                                            ]                    =                                                    [                                                                                                    w                        11                                                                                                            w                        12                                                                                    …                                                                                      w                                                  1                          ⁢                                                      N                            T                                                                                                                                                                                                  w                        21                                                                                                            w                        22                                                                                    …                                                                                      w                                                  2                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                  i                          ⁢                                                                                                          ⁢                          1                                                                                                                                    w                                                  i                          ⁢                                                                                                          ⁢                          2                                                                                                            …                                                                                      w                                                  iN                          T                                                                                                                                                ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                                              N                            T                                                    ⁢                          1                                                                                                                                    w                                                                              N                            T                                                    ⁢                          2                                                                                                            …                                                                                      w                                                                              N                            T                                                    ⁢                                                      N                            T                                                                                                                                              ]                            ⁡                              [                                                                                                                              s                          ^                                                1                                                                                                                                                                          s                          ^                                                2                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                j                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                                          N                          T                                                                                                                    ]                                      =                                          W                ⁢                                  s                  ^                                            =              WPs                                                          [                  Formula          ⁢                                          ⁢          5                ]            
In the following description, a reception signal is explained. When NR receiving antennas exist, if reception signals of the receiving antennas are set to y1, y2, . . . , yNR, a reception signal vector can be represented by Formula 6.y=[y1,y2, . . . ,yNR]T  [Formula 6]
In a wireless communication system using MIMO antenna, a channel can be represented by an index of a transmitting antenna and an index of a receiving antenna. A channel between a transmitting antenna j and a receiving antenna I can be represented by hij. This channel can be represented by a vector or matrix in a manner of tying channels between a plurality of transmitting antennas and a plurality of receiving antennas together.
FIG. 2 is a diagram of channels between a receiving antenna i and NT transmitting antennas, respectively.
Referring to FIG. 2, the channels between the receiving antenna i and NT transmitting antennas can be represented by a vector expressed as Formula 7.hiT=[hi1,hi2, . . . ,hiNT]  [Formula 7]
And, channels between NT transmitting antennas and NR relieving antennas can be expressed as the matrix shown in Formula 8.
                    H        =                              [                                                                                                                                                                    h                      1                      T                                                                                                                                        h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                                          iN                      T                                                                                                                    ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        …                                                                      h                                                                  N                        R                                            ⁢                                              N                        T                                                                                                                  ]                                              [                  Formula          ⁢                                          ⁢          8                ]            
A transmission signal passes through a channel and then has AWGN (additive white Gaussian noise) added thereto. If so, a reception signal is the signal. If white noses added to receptions signals received by NR receiving antennas are set to n1, n2, . . . , nNR, respectively, a white noise vector can be represented by Formula 9.n=[n1,n2, . . . ,nNR]T  [Formula 9]
Hence, a reception signal vector can be expressed as Formula 10.
                    y        =                              [                                                                                y                    1                                                                                                                    y                    2                                                                                                ⋮                                                                                                  y                    i                                                                                                ⋮                                                                                                  y                                          N                      R                                                                                            ]                    =                                                                      [                                                                                                              h                          11                                                                                                                      h                          12                                                                                            …                                                                                              h                                                      1                            ⁢                                                          N                              T                                                                                                                                                                                                                    h                          21                                                                                                                      h                          22                                                                                            …                                                                                              h                                                      2                            ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            h                                                      i                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                h                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                      …                                                                                              h                                                      iN                            T                                                                                                                                                              ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            h                                                                                    N                              R                                                        ⁢                            1                                                                                                                                                h                                                                                    N                              R                                                        ⁢                            2                                                                                                                      …                                                                                              h                                                                                    N                              R                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                ⁡                                  [                                                                                                              x                          1                                                                                                                                                              x                          2                                                                                                                                    ⋮                                                                                                                                      x                          j                                                                                                                                    ⋮                                                                                                                                      x                                                      N                            T                                                                                                                                ]                                            +                              [                                                                                                    n                        1                                                                                                                                                n                        2                                                                                                                        ⋮                                                                                                                          n                        i                                                                                                                        ⋮                                                                                                                          n                                                  N                          R                                                                                                                    ]                                      =                          Hx              +              n                                                          [                  Formula          ⁢                                          ⁢          10                ]            
An MIMO system can be categorized into a space multiplexing scheme and a space diversity scheme according to a presence or non-presence of the same data transmission.
The space multiplexing scheme means the scheme for transmitting data at high speed by transmitting different data via a plurality of transmitting antennas simultaneously without increasing a bandwidth. The space diversity scheme means the scheme for obtaining transmission diversity by transmitting the same data via a plurality of transmitting antennas. Space time channel coding is an example for the space diversity scheme.
The MIMO technique can be also categorized into an open loop scheme and a closed loop scheme according to a presence or non-presence of feedback of channel information to a transmitting side from a receiving side. The open loop scheme includes a space-time trellis code (STTC) scheme for obtaining a transmission diversity and coding gain using BLAST and space region capable of extending an information size amounting to the number of transmitting antennas in a manner that a transmitting side transmits information in parallel and that a receiving side detects a signal using ZF (zero forcing) and MMSE (minimum mean square error) scheme repeatedly. And, the closed loop scheme includes a TxAA transmit antenna array scheme or the like.
FIG. 3 is a conceptional diagram for CoMP (coordinated multi-point) of an intra eNB and an inter eNB according to a related art.
Referring to FIG. 3, intra eNBs 310 and 320 and an inter eNB 330 exist in a multi-cell environment. In LTE (long term evolution) system, an intra eNB is constructed with several cells or sectors. Generally, physically co-located cells are called intra cells and cells in different locations are called inter cells, respectively. A single cell MIMO user within a single cell communicates with one serving cell in one cell (sector). A multi-cell user located on a cell boundary is able to communicate with a plurality of serving base stations in a multi-cell (multi-sector).
A coordinated multi-point (CoMP) system (hereinafter abbreviated CoMP system) is the system to improve data transmission efficiency of a user located on a cell boundary by applying enhanced MIMO transmission in a multi-cell environment. If the CoMP system is applied, it is able to reduce inter-cell interference in the multi-cell environment. If the CoMP system is used, a mobile station can be provided with a support from multi-cell base stations jointly. Moreover, each base station is able to enhance system performance by supporting at least one or more mobile stations MS1, MS2, . . . MSK simultaneously using the same radio frequency resource. Namely, the base station is able to perform space division multiple access (SDMA) method based on status information of a channel between the base station and the mobile station.
The CoMP scheme can be categorized into a joint processing scheme through data sharing and a coordinated scheduling scheme/beamforming scheme.
In a CoMP system, a serving base station and at least one or more coordinated base stations are connected to a scheduler via a wired/wireless network. The scheduler is able to operate by receiving feedback of channel information on a channel status between each mobile station (MS1, MS2, . . . MSK) and the coordinated base station.
In a wireless communication system, a base station transmits downlink control information to a mobile station. The mobile station transmits uplink control information to the base station. Therefore, the demand for a method of transmitting control information in a CoMP system is ongoing to rise.